The invention concerns a press section of a paper or paperboard making machine, comprising a number of rolls which form press nips that dewater the web, the web being arranged to run through these nips, and of which rolls at least one roll is a smooth-faced so-called center roll, which forms a press nip with at least one other press roll, over which a press felt, which has been formed as an endless loop, is passed to absorb water from the web.
In a paper making machine, out of fiber pulp, a web is formed in the former of the paper machine, whereupon the formed web is passed, being supported and carried by one or more felts in the paper making machine, into the press section of the paper making machine, wherein both the web and the felts that support it are passed through nips formed by the rolls in the press section to absorb water from the web into the felts. From the press section the web is passed into the drying section of the paper machine. A conventional construction in a press section comprises a large and massive center roll as well as wire or felt loops grouped around it, the rolls placed inside these felt loops forming press nips either with one another or together with the center roll, and when the web runs through said press nips, water is drained out of the web by the effect of compression, this water being absorbed into the felts. In the drying section, water is removed from the web by means of evaporation, which is highly energy-consuming and therefore expensive and uneconomical. This is why attempts are made to remove a maximal proportion of water out of the web before it reaches the drying section, in the press section, by mechanical means.
It is known from the prior art that water is removed out of a web considerably more readily at an elevated temperature, because the viscosity of water and the springback coefficient of the web are thereby lowered together with the surface tension. Owing to this plienomenon, it has been previously found desirable to raise the temperature of the web in the press section. Based on this earlier experience, it can be established that, e.g., an increase in the temperature by 6.degree. . . . 10.degree. C. in the press section produces an increase of an order of 1% or more in the dry solids content of the web. An increased dry solids content in the press section produces considerable cost economies. For example, in paper making machines a rule of thumb is that, if the moisture content in the web in the press section can be lowered by 1%, the consumption of steam in the drying section is lowered by about 5%.
One drawback in the press sections which have been heretofore commonly used relates to the center roll in the press section. Generally, some suitable rock, such as granite, is used for the center roll. As is well known, rock rolls are quite sensitive to large and sudden changes in temperature, and the effects of such changes may be so severe as to crack the roll. This is why attempts have been made to develop suitable substitutes for granite rolls. As substitutes for rock rolls, e.g., rolls have been used which are coated, e.g., with a mixture of polyurethane and rock dust to make the surface properties of the roll similar to those of a rock roll. Advantages of metal rolls compared with rock rolls include their considerably better ability to tolerate variations in temperature. Moreover, owing to this phenomenon, they can be run at considerably higher temperatures than rock rolls. Moreover, a metal roll can be run at considerably higher running speeds than rock rolls.
A conventional construction of the press section of a paper machine wherein a center roll and a plurality of press rolls grouped around it are employed constitutes three press nips. In such a construction, the first press nip is formed between a grooved roll and a press-suction roll. In this construction, the second press nip is formed between a press-suction roll and the center roll, and a third press nip is formed between the center roll and a second grooved roll. Since, in the nips in the press section, it must be possible to make the linear loads as uniform as possible, in such a structure, as a rule, the grooved rolls are variable-crown rolls, preferably rolls whose crowns are variable in zones thereof. Thus, owing to the crown variation of the grooved rolls, in the first nip and in the third nip in the press section, a uniform linear load is achieved. In order that a linear load as uniform as possible can also be obtained for the second press nip, the mantles of the press-section roll and the center roll, which form the second press nip, are generally cambered. Because of the camber, a uniform linear load is never obtained for a nip and it is a further drawback of the cambering that the camber is always "fixed". If the camber has to be changed, the roll must be subjected to a grinding operation. This is a costly and laborious procedure. Also, cambering alone does not render the linear load profile subject to full control. For example, a problem with the metallic center rolls presently in use has been uneven heating. This has caused distortions in the linear load profile. Since the grooved rolls are provided with crown-variation means, the grooved rolls have been highly expensive. In this respect, the high cost has also been contributed to by the fact that it has been difficult to fit the crown-variation means inside a grooved roll, because the diameters of the grooved rolls are relatively small. When a fourth, separate press nip has been added to such a press section, a variable-crown grooved roll has also been used to achieve the fourth press nip.
Thus, to summarize the drawbacks of the prior art, they include high cost of construction, the aforementioned problems of uneven temperature related to metallic rolls, as well as the difficulties in providing uniform and, if necessary, adjustable profiles of linear loads in the press nips.